Perceptual and Motor Skills, 1975,41, 335-338. @ Perceptual and Motor Skills 1975

W H Y CAN'T WE TICKLE

OURSELVES?

GUY CLAXTON University of London Institute of Educatiotzl

Summrrry.-When we tickle ourselves something prevents the sensation being as strong as when someone else does it. It is suggested chat the 'something' may be: ( i ) the predictability of the stimulus; ( i i ) the presence of feedback from the movement of the arm doing the tickling; (iii) the presence of a corollary discharge from the voluntary movement of the tickling arm; ( i v ) the absence of a social/sexual context. To study these, Ss rated perceived 'tickle-strength' in situations where they were tickled: ( a ) with their eyes closed; ( b ) with their eyes open; ( c ) with their own arm doing the tickling, but being moved by someone else; ( d ) by themselves. One group of Ss was divided into same sex and different sex subject-experimenter pairs. The results showed significant effects of predictability and sensorimotor feedback. Ic is a common observation that tickling ourselves produces a much smaller effect-whether pleasant or unpleasant-than when the same stimulus is applied by someone else. Why should this be so? Weiskrantz, Elliot, and Darlington (1971) considered the role of two 'physiological' factors-feedback and reafference. When we are tickled by another person, we have no feedback from our own limbs that correlates with the stimulus. Neither, since we do not initiate any voluntary activity, do we have a motor command to the limb nor a corollary discharge (von Holsc, 1954) of the command. When we tickle ourselves the suppression of the effect could be due to the presence of either a cancelling feedback component or a corollary discharge ('reafference') component. Weiskrantz et al.'s study implicated both of these. The present study sought to extend these findings by introducing two new variables, feedback and reafference being again studied. The first is the p7edictability of the stimulus. When someone else tickles us, the place and time of onset of the stimulus is uncertain: when we tickle ourselves, it is not. Second, common sense suggests a strong social/sexzlal inflaence on perceived 'tickle-strength' so in the experiment subjects were tickled by members of the same or of the opposite sex. To study these variables, four main conditions were used. In the first (Condition 1) subjects were tickled by someone else while their eyes were closed. Here the stimulus was unpredictable and neither feedback nor reafferent information was available. The second (Condition 2 ) was the same but with the subject's eyes open. This differs from the first only in that the stimulus is relatively predictable. In Condition 3, the subject held the tickling stimulus 'Department of Child Development and Educational ~s~chology, 24 and 27 Wodburn Square, London WClH OAA, England.

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G . CLAXTON

( a feather) with his eyes open, but it was applied by someone else moving his hand in the appropriate way. Now the stimulus is predictable and feedback is present from the arm's movement, but, as the movement is not voluntary, there is no motor command and hence no reafference. In the final condition (Condition 4 ) subjects tickled themselves, thus having predictability, feedback, a d reafference. By measuring the rated tickle-strength in each condition we may infer which of these factors underlie the self-tickling effect.

MTHOD Subjects were 49 students attending a laboratory class, enrolled for Dip.Ed. or M.A. courses in the Psychology of Education at the University of London Institute of Education. Their ages ranged from 25 to 45 yr., and men and women were approximately equally represented. The experiment was conducted in two sessions of 30 and 19 students (Groups 1 and 2 ) . The procedure was as follows. The students were divided into small groups of about three or four. The tickling stimulus was delivered to the bared forearm with a feather, and in each condition comprised five strokes with the feather lasting about 5 sec. in all. The students were asked to make the strokes as light, i.e., as ticklish, as possible. While this informal method introduces a risk of error, it was thought a worthwhile one to take in order to get a real-life, genuinely ticklish stimulus. In Weiskrantz, et al.'s study the stimulus was a knitting-needle tip applied to the skin with a pressure of about 17 gm; this seemed very heavy for a tickle. It would cerrainly stimulate sub-cutaneous pressure receptors which a natural tickle would not. The inside of the forearm was chosen as the area to be tickled, as a reasonable compromise between sensitivity and intimacy. There seems to be no good reason why the results should not be generalizable to other areas of the body surface. After being tickled, Ss were asked to rate the ticklishness on a scale from 0 ('not at all ticklish') to 7 ('very ticklish indeed'). All Ss performed in all four conditions, with the order of the conditions being counterbalanced across Ss by a 4 X 4 randomized Latin square to control for sequential effects such as habituation, boredom, sensitization, etc., and sequential effects between the conditions. Within each experimental session a quarter of the Ss were assigned to each of the four random orders (5, 5, 5 and 4 in the case of the second group). Each S was tickled twice in each condition, once in the sequence specified by the Latin square and again (after a break) in the reverse sequence. All students acted as both subject and experimenter. RESULTS The mean score of each S's two ratings in each condition was computed, and the total data subjected to a two-way analysis of variance with conditions

WHY CAN'T WE TICKLE OURSELVES?

337

and subjects as factors. The effect of conditions was significant (F3,185 = 45.9, p < .001). Post hoc Scheffk tests were used to compare the means in a pairwise fashion, and these yielded the following ordering in terms of rated ticklestrength: Condition 1 > Condition 2 > Condition 3 = Condition 4. The means and standard deviations are shown in Table 1. For the second group of 19 Ss, the data were divided according to whether each particular subject-experimenter pair contained members of the same or different sex. t tests for comparison of the means in each condition indicated no significant differences. TABLE 1

MEANTICKLISHNESS RATINGS IN FOURCONDITIONS,DIVIDED FOR GROUP 2 TO SHOW EFFECTOP BEINGTICKLED BY A PERSON OF SAMEOR OPPOSITE SEX Group Group 2 Both Groups

Sex same different M SD

Condition 1

2

3.25 3.72 3.71 1.39

2.80 3.22 2.85 1.24

n 3

4

1.95

1.90 2.17 1.71 0.85

2.39 2.13 0.89

10

9 49

DISCUSSION The results for the different conditions can best be understood by comparing Conditions 1 and 2, 2 and 3, and 3 and 4. The only difference between the first two conditions was that in Condition 2 S s eyes were open, while in 1 they were closed. It is reasonable to attribute the greater ticklishness in Condition 1 to the uncertainty that S has about the precise time and place of stimulus onset. Intuitively, he does not have a chance to 'steel himself' against the sensation in the one case. More formally, i t seems that there exists a high-level, perhaps even conscious, ability to control the perceived magnitude of sensation. This probably occurs in other modalities too; an unexpected noise seems louder than an expected one. The difference between Conditions 2 and 3 is in the presence, in the latter case, of feedback from movement receptors in the subject's arm. The significantly greater sensation in the no-feedback case confirms Weiskrantz, et al.'s finding that feedback is instrumental in inhibiting somatosensory perception. Conditions 3 and 4 differ in the presence in the latter of a corollary discharge within the subject due to voluntary control of movement in the arm holding the feather. The present results differ from Weiskrantz, et al.'s in showing only a small, insignificant effect. While we cannot conclude that reafference is not playing an inhibitory role here, there is psimu facie evidence that 'the stability of the tactile world,' as represented by the self-tickling effect may not be mediated by entirely the same mechanisms as account for 'the

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G.CLAXTON

stability of the visual world' where reafferent commands have been strongly implica ted. Two suggestions for the difference between the two sets of results may be made. Possibly it is due to the different nature and pressure of the stimulus. Reafferent mechanisms may only become operative when the afferent stimulus contains pressure or deformation information. The second striking difference is that the present study involves a direct social interaction between tickler and tickled, which the earlier one, using a mechanically administered stimulus, lacked. Although it is difficult to see how this could have produced the specific discrepancy, a preliminary look at this was taken by comparing the effect of sex of subject and of experimenter being the same or different. In fact, none of the sex differences is significant, although there is a consistent trend toward greater ticklishness in mixed sex pairs. If this factor were an important one, we might at least have expected an increased 'sex effect' in Condition 3, where the members of the pair are in actual physical contact. REFERENCES Relations between the central nervous system and peripheral organs. Brit. I. anim. Behav., 1954, 2 , 89-94. WEISKRANTZ,L., ELLIOT,J., & DARLINGTON, C. Preliminary observations in tickling oneself. Natuse, 1971, 230, 598-599.

HOLST,E.

V.

Accepted June 1 7, 1975.

Why can't we tickle ourselves?

When we tickle ourselves something prevents the sensation being as strong as when someone else does it. It is suggested that the "something" may be: (...
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